Property dispute

my father buy a property in 1973 in the name of my grandmother before marriage, G.P.A. registered with a Sub-registrar office in Delhi.
but G.P.A. had signed by my father, no signature & thumb impression of my grand mother on G.P.A.
now my father & my grand mother is died in 2011. now my father's brothers
(other sons of my grandmother) ask for their share in this property 
pl advice..